Familiar Letters on Chemistry eBook

LETTER IX

My dear Sir,

The facts detailed in my last letter will satisfy
you as to the manner in which the increase of mass
in an animal, that is, its growth, is accomplished;
we have still to consider a most important question,
namely, the function performed in the animal system
by substances destitute of nitrogen; such as sugar,
starch, gum, pectine, &c.

The most extensive class of animals, the graminivora,
cannot live without these substances; their food must
contain a certain amount of one or more of them, and
if these compounds are not supplied, death quickly
ensues.

This important inquiry extends also to the constituents
of the food of carnivorous animals in the earliest
periods of life; for this food also contains substances,
which are not necessary for their support in the adult
state. The nutrition of the young of carnivora
is obviously accomplished by means similar to those
by which the graminivora are nourished; their development
is dependent on the supply of a fluid, which the body
of the mother secretes in the shape of milk.

Milk contains only one nitrogenised constituent, known
under the name of caseine; besides this, its chief
ingredients are butter (fat), and sugar of milk.
The blood of the young animal, its muscular fibre,
cellular tissue, nervous matter, and bones, must have
derived their origin from the nitrogenised constituent
of milk—­the caseine; for butter and sugar
of milk contain no nitrogen.

Now, the analysis of caseine has led to the result,
which, after the details I have given, can hardly
excite your surprise, that this substance also is
identical in composition with the chief constituents
of blood, fibrine and albumen. Nay more—­a
comparison of its properties with those of vegetable
caseine has shown—­that these two substances
are identical in all their properties; insomuch, that
certain plants, such as peas, beans, and lentils, are
capable of producing the same substance which is formed
from the blood of the mother, and employed in yielding
the blood of the young animal.

The young animal, therefore, receives in the form
of caseine,—­which is distinguished from
fibrine and albumen by its great solubility, and by
not coagulating when heated,—­the chief constituent
of the mother’s blood. To convert caseine
into blood no foreign substance is required, and in
the conversion of the mother’s blood into caseine,
no elements of the constituents of the blood have been
separated. When chemically examined, caseine is
found to contain a much larger proportion of the earth
of bones than blood does, and that in a very soluble
form, capable of reaching every part of the body.
Thus, even in the earliest period of its life, the
development of the organs, in which vitality resides,
is, in the carnivorous animal, dependent on the supply
of a substance, identical in organic composition with
the chief constituents of its blood.